[dpdk-dev] [PATCH 2/2] net/virtio: optimize header reset on any layout

Maxime Coquelin maxime.coquelin at redhat.com
Wed Jan 11 09:01:21 CET 2017



On 01/11/2017 05:27 AM, Yuanhan Liu wrote:
> When any layout is used, the header is stored in the head room of mbuf.
> mbuf is allocated and filled by user, means there is no gurateen the
> header is all zero for non TSO case. Therefore, we have to do the reset
> by ourself:
>
>     memest(hdr, 0, head_size);
>
> The memset has two impacts on performance:
>
> - memset could not be inlined, which is a bit costly.
> - more importantly, it touches the mbuf, which could introduce severe
>   cache issues as described by former patch.
>
> Similiary, we could do the same trick: reset just when necessary, when
> the corresponding field is already 0, which is likely true for a simple
> l2 forward case. It could boost the performance up to 20+% in micro
> benchmarking.
>
> Cc: Maxime Coquelin <maxime.coquelin at redhat.com>
> Cc: Michael S. Tsirkin <mst at redhat.com>
> Cc: stable at dpdk.org
> Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
> ---
>  drivers/net/virtio/virtio_rxtx.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index 8ec2f1a..5ca3a88 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -292,8 +292,14 @@
>  		hdr = (struct virtio_net_hdr *)
>  			rte_pktmbuf_prepend(cookie, head_size);
>  		/* if offload disabled, it is not zeroed below, do it now */
> -		if (offload == 0)
> -			memset(hdr, 0, head_size);
> +		if (offload == 0) {
> +			ASSIGN_UNLESS_EQUAL(hdr->csum_start, 0);
> +			ASSIGN_UNLESS_EQUAL(hdr->csum_offset, 0);
> +			ASSIGN_UNLESS_EQUAL(hdr->flags, 0);
> +			ASSIGN_UNLESS_EQUAL(hdr->gso_type, 0);
> +			ASSIGN_UNLESS_EQUAL(hdr->gso_size, 0);
> +			ASSIGN_UNLESS_EQUAL(hdr->hdr_len, 0);
> +		}
>  	} else if (use_indirect) {
>  		/* setup tx ring slot to point to indirect
>  		 * descriptor list stored in reserved region.
>

Reviewed-by: Maxime Coquelin <maxime.coquelin at redhat.com>

Thanks!
Maxime


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